1. Field of the Invention
The present invention relates to a phase-changeable optical information recording medium on which information can be recorded, reproduced and rewritten in high density and with high speed by using optical techniques such as irradiation of laser beams, and a method for recording and reproducing information thereon.
2. Description of the Prior Art An optical magnetic recording medium or a phase-changeable recording medium is known as a medium on which information can be recorded in large capacity and reproduced and rewritten at a high speed. A transportable optical recording medium is expected to be more important in a highly information-oriented society. The improvements in the function of applications and in the quality of graphic information require of the medium larger capacity and higher-speed recording or reproducing.
These optical recording media utilize a difference in the optical characteristics of the recording material created by locally irradiating the recording material with laser beams. For example, for the optical magnetic recording medium, a difference in the angle of rotation in a plane of polarization of a reflected light, which is created by a difference in the magnetized state, is utilized for recording, Furthermore, for the phase-changeable recording medium, an amount of reflected light in the crystalline state different from that in the amorphous state when light with a specific wavelength is used is utilized for recording. The phase-changeable recording medium is advantageous because the erasure of recorded information and the overwriting of information can be performed simultaneously by adjusting the output power of the laser, so that it is possible to rewrite information signals at high speed.
FIG. 7 shows a typical layer structure of a conventional optical information recording medium. A resin such as polycarbonate, polymethyl methacrylate (hereinafter, referred to as PMMA) or glass can be used for a substrate 101. The substrate 101 includes a guide groove for guiding laser beams. Protective layers 102 and 104 will be described later. A recording layer 103 is formed of a material that has different optical characteristic states and can change between the different states reversibly. In the case of a rewritable phase-changeable optical recording material, the recording layer 103 can be formed of a so-called chalcogenide such as a material containing Te or Se as a main component, or a material containing Sb such as materials comprising Texe2x80x94Sbxe2x80x94Ge, Texe2x80x94Snxe2x80x94Ge, Texe2x80x94Sbxe2x80x94Gexe2x80x94Se, Texe2x80x94Snxe2x80x94Gexe2x80x94Au, Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te, Inxe2x80x94Sbxe2x80x94Se, Inxe2x80x94Texe2x80x94Se or the like as a main component. A reflection layer 105 generally is formed of a metal such as Au, Al, Cr or the like, or an alloy of these metals. The reflection layer 105 is provided for the purpose of radiating heat and allowing the recording thin film to absorb light effectively, but the reflection layer need not be provided. For the purpose of preventing the oxidation or the corrosion of the optical information recording medium or the attachment of dust onto the medium, an overcoat layer or a dummy substrate may be formed on the reflection layer 105, although these are not shown in FIG. 7. The dummy substrate may be bonded with an ultraviolet curing resin.
Furthermore, as shown in FIG. 8, a recording medium comprising two protective layers 102 and 106 between the substrate 101 and the recording layer 103 has been proposed. For example, Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 5-217211 discloses a recording medium comprising a recording layer containing Ag, a first protective layer formed of a nitride of SiN or AlN or a carbide of SiC in contact with the recording layer and a second protective layer formed of ZnS or a composite compound containing ZnS on the first protective layer. The first protective layer is formed to suppress a reaction between a constituent S atom of the second protective layer and a constituent Ag atom of the recording layer. Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 6-195747 discloses another example of the recording medium comprising protective layers in a two-layered structure, as shown in FIG. 8. The recording medium includes a first and second protective layers 106 and 102 between a recording layer 103 and a substrate 101. The first protective layer 106 in contact with the recording layer 103 is formed of Si3N4, and the second protective layer 102 in contact with the substrate 101 is formed of ZnSxe2x80x94SiO2.
The protective layers 102, 104 and 106 serve to protect the recording layer 103 in such a manner that the material for the recording layer 103 is prevented from being oxidized, evaporated or distorted. Furthermore, it is possible to adjust the absorption of the optical information recording medium or a difference in the reflectance between a recorded portion and an erased portion by adjusting the thickness of the protective layers. Thus, the protective layers also serve to adjust the optical characteristics of the medium. Moreover, a material for the protective layers 102, 104 and 106 is required to have good adhesiveness with a material forming the recording layer and the substrate 101 and good weather resistance so that the protective layers 102, 104 and 106 are not cracked. When the protective layers 102, 104 and 106 are used in contact with the recording layer 103, the material for the protective layers is required not to impair the optical change of the material for the recording medium. Examples of the material for the protective layers 102, 104 and 106 include a dielectric such as a sulfide such as ZnS, an oxide such as SiO2, Ta2O5 or Al2O3, a nitride such as GeN, Si3N4 or Al3N4, a nitrogen oxide such as GeON, SiON or AlON, a carbide, a fluoride or the like, or suitable combinations thereof
It is known conventionally that when information is rewritten, a marked position is dislocated slightly after rewriting, namely, a so-called overwrite distortion (distortion in the recorded mark) is caused. This distortion is caused because the temperature increase rate during laser irradiation depends on whether the recording layer was in an amorphous state or a crystalline state before rewriting, whereby a mark after rewriting has a length that does not match a predetermined length. In order to solve this problem, so-called absorption correction is performed by maintaining Ac/Aa in a predetermined range larger than 1, where Aa represents the absorptance in the amorphous portion and Ac represents the absorptance in the crystalline portion. The structure that allows such absorption correction makes the increase in the temperature in marked portions uniform so that the mark distortion is unlikely to occur at rewriting.
For example, Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 7-78354 discloses an information recording medium comprising a metal layer, a protective layer, a recording layer and a reflection layer on a substrate in this order, in which the reflectance after recording is larger than that before recording. Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 7-105574 discloses an optical information recording medium comprising an optical absorption layer formed of Ti on a substrate, in which an optical absorptance in the crystalline state in the recording layer is larger than that in the amorphous state to reduce the dislocation of recorded marks.
Especially when information is rewritten at high speed, the overwrite distortion as described above is caused readily. However, the solution of merely maintaining Ac/Aa larger than 1 cannot provide a sufficient erasure ratio. In addition, when a recording layer composition that allows a high rate of crystallization is used to raise the erasure ratio, it is difficult to obtain sufficient reliability of recorded signals.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical information recording medium that allows high speed recording of information resulting from a high crystallization rate while suppressing the overwrite distortion, and a method for recording and reproducing information thereon. It is another object of the present invention to provide an optical information recording medium that provides signals recorded thereon with high reliability even if the crystallization rate is high and a method for recording and reproducing information thereon.
An optical information recording medium of the present invention includes a recording layer that changes reversibly between a crystalline state and an amorphous state. The irradiation of the recording layer with laser beams at a predetermined wavelength changes the recording layer from one selected from the group consisting of the crystalline state and the amorphous state to the other state. The absorptance Ac of the laser beams in the recording layer when the recording layer is in the crystalline state is larger than the absorptance Aa of the laser beams in the recording layer when the recording layer is in the amorphous state. Crystallization accelerating layers for accelerating the change of the recording layer from the amorphous state to the crystalline state are formed in contact with both surfaces of the recording layer.
This embodiment can provide a medium having a high erasure ratio at high speed rewriting.
A method for recording and reproducing optical information of the present invention is used with the optical information recording medium including a recording layer that changes reversibly between a crystalline state and an amorphous state. The irradiation of the recording layer with laser beams at a predetermined wavelength changes the recording layer from one selected from the group consisting of the crystalline state and the amorphous state to the other state. The absorptance Ac of the laser beams in the recording layer when the recording layer is in the crystalline state is larger than the absorptance Aa of the laser beams in the recording layer when the recording layer is in the amorphous state. Crystallization accelerating layers for accelerating to change the recording layer from the amorphous state to the crystalline state are formed in contact with both surfaces of the recording layer. The method includes the steps of focusing laser beams on a microspot on the optical information recording medium by an optical system, and irradiating the recording layer with the laser beams. Apower level of the laser beams is fluctuated between a first power level P1 and a second power level P2 to change the optical information in the recording layer, and the change of the optical information is selected from the group consisting of recording, erasure and overwriting. The optical information is reproduced with the laser beams of a third power level P3. Herein, the first power level P1 is an amorphous state-formation level that allows a local portion in the recording film to change reversibly from the crystalline state to the amorphous state by irradiation of the laser beams. The second power level P2 is a crystalline state-formation level that allows a local portion in the recording film to change reversibly from the amorphous state to the crystalline state by irradiation of the laser beams. The third power level P3 is a reproduction level that is lower than the power levels P1 and P2. The irradiation of the laser beams at P3 does not affect the optical state of the recording layer. The irradiation of the laser beams at P3 provides a sufficient reflectance to reproduce optical information.
This embodiment allows recording and reproducing of information signals at high speed while suppressing overwrite distortion.
As described above, by forming crystallization accelerating layers in contact with the recording layer whose optical characteristic changes reversibly and achieving Ac greater than Aa, the optical information recording medium allows signals to be rewritten at high speed and has excellent thermal stability of recorded marks and excellent characteristics in repetitive recording. Moreover, the method of the present invention maximizes the performance of the optical information recording medium of the present invention sufficiently.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.